An internal combustion engine typically includes a plurality of engine valves. These engine valves control the intake and exhaust of gases relative to the combustion chamber(s) of the engine. A typical engine will include at least one intake valve and at least one exhaust valve for each combustion chamber of the engine. The opening of each valve is timed to occur at a predetermined cam or crank shaft angle in the operating cycle of the engine. For example, an intake valve may be opened when a piston is moving from a top-dead-center position to a bottom-dead-center position in its cylinder to pass air into the combustion chamber. The exhaust valve may be opened during the movement of the piston toward top-dead-center to expel an exhaust gas from the combustion chamber.
The actuation, or opening and closing, of the engine valves may be achieved in a number of ways. For example, the engine may drive a crankshaft that is rotatively connected to a cam shaft. Each engine valve may be mechanically actuated by this cam shaft. In addition, the rotation of the crankshaft also may control the reciprocal motion of the combustion chamber piston. Thus, the rotation of the crankshaft mechanically controls and coordinates the timing of actuation of each engine valve with the desired movements of the respective combustion chamber piston.
Mechanically actuating the engine valves, however, provides no flexibility in the timing of valve actuation. It has been found that engine operating characteristics, for example, efficiency, may be improved by varying the timing of the valve actuation based on the operating parameters of the vehicle. With mechanical actuation, the engine valves will be actuated at the same timing angle of crankshaft rotation regardless of the vehicle operating parameters. Thus, these types of inflexible systems may not be capable of optimizing engine performance.
Another approach involves actuating the engine valves independently of the crankshaft rotation. This may be accomplished, for example, with a hydraulic system. As shown in U.S. Pat. No. 6,263,842 to De Ojeda et al., dated Jul. 24, 2001, a hydraulically-driven piston may be used to actuate an engine valve. In this approach, a hydraulic piston is connected to each engine valve and is actuated by the introduction of pressurized fluid. The actuation of the engine valve may, therefore, be controlled independently of the crankshaft rotation and may provide additional flexibility in the valve timing.
To obtain further improvements in engine efficiency, the engine valves may need to be actuated when the gas within the combustion chamber is under pressure. A hydraulically-actuated engine valve, as discussed above, will need to exert a significant force to open the engine valve under these conditions. This may require either a highly pressurized fluid or a valve actuation piston with a large surface area. An additional pump may be required to provide the highly pressurized fluid.
In addition, the hydraulically-actuated engine valve discussed above may not be able to accurately control the amount of engine valve movement during actuation. In a situation where the engine valve is actuated when the combustion chamber piston is advancing within the combustion chamber, the amount of engine valve lift may need to be limited to prevent a collision between the combustion chamber piston and the engine valve. Such a collision may damage the engine valve and prevent the engine valve from properly sealing the gas passageway. This damage may disrupt the operation of the engine.
Furthermore, the hydraulically-actuated valve discussed above may not be able to control the speed of the engine valve during engine valve actuation. Seating an engine valve at high velocity may result in high seating forces that damage the engine valve or the valve seat, thereby preventing the engine valve from properly sealing and reducing the efficient operation of the engine.
In addition, if a high-force hydraulically-actuated engine valve requires a valve actuation piston with a large surface area, a substantial amount of highly pressurized fluid could be required each time the engine valve is actuated. This could significantly decrease the amount of fluid available to other high pressure systems within the vehicle. Moreover, it would be beneficial to recycle at least a part of this highly pressurized fluid so that some of the hydraulic energy used to pressurize this fluid may be recuperated, thereby increasing engine efficiency and reducing parasitic losses.
The valve actuation system of the present invention solves one or more of the problems set forth above.